Coefficient of thermal expansion facts for kids
Solids usually get bigger when they get hot and shrink when they cool down. This change in size due to temperature is called thermal expansion. The way we measure how much something expands or shrinks is with its coefficient of thermal expansion.
This "expansion number" helps us understand:
- How much a material's length changes (linear thermal expansion).
- How much a material's surface area changes (area thermal expansion).
- How much a material's total size or volume changes (volumetric thermal expansion).
These different ways of measuring expansion are all connected. We can measure how much the volume changes for almost anything, like liquids and solids. But measuring how much the length changes (linear expansion) is mostly for solid things. This is very important in engineering, which is about designing and building things.
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How Different Materials Expand
It's really important to think about how materials expand and shrink when building big structures like bridges. It also matters when using measuring tapes for land surveys, or when making molds for hot materials. Basically, whenever big temperature changes are expected, we need to consider thermal expansion.
The "expansion number" (called alpha, or α) can be very small for hard solids and much larger for liquids. This number changes with temperature, and some materials change a lot! The table below shows some values for common materials. These numbers tell you how much a material expands for every degree Celsius (or Kelvin) it gets hotter.
| Coefficient of linear thermal expansion α | |
|---|---|
| Material | α in 10-6/K at 20 °C |
| Mercury | 60 |
| BCB | 42 |
| Lead | 29 |
| Aluminum | 23 |
| Brass | 19 |
| Stainless steel | 17.3 |
| Copper | 17 |
| Gold | 14 |
| Nickel | 13 |
| Concrete | 12 |
| Iron or Steel | 11.1 |
| Carbon steel | 10.8 |
| Platinum | 9 |
| Glass | 8.5 |
| GaAs | 5.8 |
| Indium Phosphide | 4.6 |
| Tungsten | 4.5 |
| Glass, Pyrex | 3.3 |
| Silicon | 3 |
| Invar | 1.2 |
| Diamond | 1 |
| Quartz, fused | 0.59 |
Real-Life Uses of Thermal Expansion
Thermal expansion is used in many clever ways! For example, you can see it in:
- Bi-metal strips: These are made of two different metals joined together. Since they expand at different rates, the strip bends when heated. This bending can be used to turn things on or off, like in a thermostat.
- Mercury thermometers: The mercury inside expands when it gets hot, moving up the tube to show the temperature.
Another cool use is in mechanical applications, like when fitting parts together. Imagine you have a metal ring (called a bushing) that needs to fit tightly onto a rod (a shaft). You can make the ring's hole slightly smaller than the rod. Then, you heat the ring, which makes its hole expand. Once it's hot and expanded, you can easily slide it onto the rod. As the ring cools down, it shrinks and grips the rod very tightly. This is called a 'shrink fit'.
Some special metal mixes, called alloys, are designed to expand very little, even with big temperature changes. One example is Invar 36. These alloys are super useful in places like spacecraft, where temperatures can swing wildly from very hot to very cold. They help keep parts from changing size too much.
Images for kids
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Expansion joint in a road bridge used to avoid damage from thermal expansion.
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Change in length of a rod due to thermal expansion.
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Thermal expansion of long continuous sections of rail tracks is the driving force for rail buckling. This phenomenon resulted in 190 train derailments during 1998–2002 in the US alone.
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Drinking glass with fracture due to uneven thermal expansion after pouring of hot liquid into the otherwise cool glass
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Expansion loop on heating pipeline
See also
In Spanish: Coeficiente de dilatación para niños
